Quality assurance in hospital care increasingly focuses on evaluation of outcome. Problems arise with displaying results of medical care beyond discharge. In this context hospital readmissions are often used as outcome variable. But it is unclear whether readmissions are meaningful indices of quality of hospital care and if so, where to get valid data on readmissions. We used claims data of the regional health insurance fund in Saxony-Anhalt (AOK Saxony-Anhalt) from 2002 and 2003 (850,000 insured; nearly 300,000 cases per year). All hospital admissions of a insured person are identified by an anonymous id-number independent of the admitting hospital. By this way we can analyze readmissions individually. Readmission are frequent events in hospital care. Nearly one third of all patients were admitted at least a second time in 2003. 18 % of all hospital cases are readmissions within 30 days after discharge. Readmissions concentrated on chronically ill, oncological, or multimorbid patients. Many of the readmissions take place in the context of planned therapies or post-operative treatment. 'Revolving-door patients' with multiple readmissions point to problems in cooperation of ambulatory and hospital care. By defining tracer diagnoses and specific causes of readmissions unplanned readmissions may be identified as a quality indicator of suboptimal care. Readmissions don't express suboptimal care per se. But taking into account methodological aspects a tracer approach with defining specific unplanned readmissions may provide meaningful outcome indicators. These can be derived from claims data fast, routinely, and with low costs. Further validation of the approach is needed.